The present invention relates to a semiconductor switch and a method for determining a current between an input terminal and an output terminal of a semiconductor switch.
For measuring and evaluating a current through a semiconductor switch, such as through an IGBT module (IGBT=insulated gate bipolar transistor), for example, it is known to provide the semiconductor switch with an additional sense terminal. In this case, the current provided at said sense terminal is approximately proportional to the main current through the semiconductor switch.
The European patent application EP 0 467 681 A2 discloses a drive circuit for such an IGBT comprising a sense terminal for determining an emitter current. On the basis of the current intensity thus determined, a control voltage at the IGBT is adapted in order to avoid overcurrents.
Since the relationship between collector-emitter current and collector-emitter voltage of such an IGBT is nonlinear, the transfer ratio between emitter current and sense current is also approximately constant only if the voltage between collector and emitter is identical to the voltage between collector and sense terminal. In order to set these required voltage ratios correctly, the sense terminal is loaded by a suitable compensation circuit with a current such that the collector-emitter voltage and the collector-sense voltage are equal in magnitude.
In this case, it is a challenge to find a suitable translation ratio between emitter current and sense current. If, proceeding from very high maximum or peak currents, a relatively large translation ratio is chosen, then the required accuracy in the case of small currents can be achieved only to a limited extent since disturbance effects of the IGBT, offset errors of the evaluation circuit and tolerances of the components have a relatively large influence in this case. By contrast, in the case of a small translation ratio, although a high accuracy can be achieved for low currents, the compensation current at the sense terminal also increases. Furthermore, even very high maximum currents may possibly no longer be measured correctly since the current output signal can encounter a limitation in this case.
Therefore, there is a need for a semiconductor switch whose power current can be determined reliably and efficiently over a large dynamic range.